The spinothalamic tract has been implicated as an important pain pathway which transmits nociceptive information during myocardial ischemia. The objective of this research is to define further the central organization of the spinothalamic tract with respect to afferent input from the heart by employing electrophysiological techniques. The upper thoracic segments of cats and monkeys will be explored using glass microelectrodes to locate the cells or axons of origin of the spinothalamic tract. These cells or axons will be antidromically activated by stimulating their axons in the contralateral or ipsilateral diencephalon, respectively. After the cells have been identified, the cardiac afferent input to these cells will be examined by electrically stimulating the afferent nerves, mechanically exciting the receptive fields and occluding the left coronary artery. All receptors in the heart have the possibility of transmitting information related to nociceptive stimuli, because the heart undergoes many changes during ischemia of the myocardium. The interaction between visceral and somatic input onto the spinothalamic tract cells will be determined by stimulating the afferent nerves from the heart and the thorax and forelimb. According to the convergence-projection theory of referred pain, the information obtained should have relevance to the understanding of angina pectoris. The splanchnic input onto the spinothalamic tract cells at this segmental level of the spinal cord also will be examined because the complaint of indigestion is sometimes associated with ischemic heart disease.